Olefins, such as ethylene and propylene, and their non-polymeric derivatives, such as isopropyl alcohol and cumene, account for some of the most demanded chemicals in the world. For example, the United States alone produces more than 10 billion pounds of chemicals derived from propylene annually.
Olefins are commonly produced by cracking hydrocarbon feedstocks or catalytically converting oxygenate feedstocks. Traditional methods for cracking include steam cracking, whereby naphtha or other hydrocarbons are reacted with steam to make light olefins, and fluid catalytic cracking (FCC), which is the refinery operation that breaks down larger hydrocarbons to produce naphtha-light components for gasoline, as well us olefins and heating oils. The conventional conversion of oxygenate feedstocks includes methanol-to-olefin (MTO) and methanol-to-propylene (MTP) processes. In MTO, methanol is converted primarily to ethylene and propylene in the presence of a molecular sieve catalyst. In MTP, methanol is dehydrated to produce dimethyl ether, which is then converted to propylene. Both processes involve complex operations downstream of the reactor(s) to purify the product, capture unconverted reagents for recycle, and purge contaminants. Typically, low temperature partial condensation is involved, and at least a portion of the uncondensed gas is recycled in the process.
In non-polymeric olefin derivative manufacturing, an olefin and other reagents are introduced into a high-pressure reactor. The raw effluent from the reactor is transferred continuously to one or more separation steps, from which a stream of raw derivative product is withdrawn for further purification. A stream of overhead gases, containing unreacted olefin, is also withdrawn from the separation steps and is recirculated back to the reactor.
Both of these types of manufacturing operations need to vent a portion of uncondensed gas to prevent build-up of unwanted contaminants in the reaction loop. However, the vented overhead gas typically contains unreacted olefin that, without further treatment, would otherwise go to waste.
Additionally, polyethylene (PE) and polypropylene (PP) are two of the most demanded polymers in the world. Together, these polymers make up half of the volume of plastic produced worldwide.
During polyolefin production, a small portion of the olefin feedstock is lost through raw material purification, chemical reaction, and product purification and finishing. In particular, paraffin that enters with the olefin feedstock must be removed to prevent its build up in the reactor loop, and olefin is lost when this paraffin is purged from the loop. This results in an annual loss of $1 million to $3 million per year for a typical polyolefin plant. The development of a more efficient way to prevent the loss of olefin monomer in the feedstock has been an on-going process for those in the petrochemical field.
In polyolefin manufacturing, a feedstock containing olefin monomer, catalysts, and other agents is introduced into a high-pressure polymerization reactor. During the reaction, a raw polymer product is produced. The raw product contains polyolefin, significant amounts of unreached olefin, and small amounts of solvents, catalysts, stabilizers, other hydrocarbons or any other materials, depending on the manufacturing process used. To remove the volatile contaminants dissolved in the raw product, it is passed to large bins, where nitrogen is used to purge them out. The vent gas from this step contains nitrogen, unreacted olefin monomer, unwanted analogue paraffins that entered with the olefin feedstock, and other process-specific materials. In the past, this vent gas was sent for flaring, resulting in a waste of unreacted olefin.
Various process and techniques have been proposed for mitigating the loss of unreacted olefin in a variety of streams.
U.S. Pat. No. 4,623,704, to Dembicki et al. (Dow Chemical Company), discloses a process for treating a polymerization vent gas with a membrane. The vent stream is compressed and then cooled and condensed. Cooled gas and liquid are sent to a liquid/gas separator. After separation, the gas stream is sent through a series of membrane separation steps, which produce a permeate stream enriched in ethylene. The recovered ethylene is recycled to the polymerization process.
Co-owned U.S. Pat. Nos. 5,089,033 and 5,199,962, to Wijmans (Membrane Technology and Research, Inc.), disclose processes for recovering a condensable component in a gas stream that would otherwise be discharged into the atmosphere. The processes involve a condensation step and a membrane separation step. In one embodiment, the gas stream is compressed and cooled to carry out the condensation step. Uncondensed gas is then passed across a membrane that is selectively permeable to the condensable component.
Co-owned U.S. Pat. No. 6,271,319, to Baker et al. (Membrane Technology and Research, Inc.), discloses a process for treating the uncondensed gas stream using a gas separation membrane that is permeable for propylene over propane. A permeate stream enriched in olefin is withdrawn and recycled to the reactor inlet.
These patents, and other prior art technologies, have mainly focused on condensing a gas stream and recovering unreacted olefin from the resulting uncondensed gas produced from the condensation step. However, little is taught on recovering olefins from the effluent stream in a membrane separation pretreatment step prior to undergoing the main compression, condensation, and membrane separation steps as in prior art processes.
Co-owned U.S. Pat. No. 5,769,927, to Gottschlich et al. (Membrane Technology and Research, Inc.), discloses a process for treating a purge vent stream from a polymer manufacturing operation. The purge vent stream contains an unreacted olefin monomer and nitrogen. The purge vent stream is initially treated in a condensation step. The uncondensed gas is then passed to a membrane separation step, where the membrane is organic-selective, meaning that the membrane is selective for unreacted monomer over other gases. The liquid condensate is directed to a flash evaporation step. The flashing step produces a liquid product stream enriched in monomer and a flash gas that is recirculated in the process.
Despite the above improvements, there remains a need for better olefin recovery technology applicable to processes that make or use olefins or olefin derivatives.